FH_Normal - check Fragment Reassembly (normal order)
Host and Router
FH_Normal.seq [-tooloption ...] -pkt Fragment.def -tooloption : v6eval tool option
1. Ping to Target (create Neighbor Cache Entries, if not exist) 2. Override Neighbor Cache Entries
Tester Target | | |-------------------------->| | Echo Request (1st) | | | | | |-------------------------->| | Echo Request (2nd) | | | | | |<--------------------------| | Echo Reply | | | | | v v
1. Send Echo Request (1st fragment) 2. Send Echo Request (2nd fragment) 3. Receive Echo Reply
Echo Request Data (original) is:
IPv6 Header Version = 6 Traffic Class = 0 FlowLabel = 0 PayloadLength = 1032 NextHeader = 58 (ICMP) SourceAddress = Tester Link Local Address DestinationAddress = Target Link Local Address
ICMP Echo Request Type = 128 (Echo Request) Code = 0 Checksum = (auto) Identifier = (auto) SequenceNumber = 0 PayloadData = data repeat{0x1, 512} data repeat{0x2, 512}
Echo Request Data (1st fragment) is:
IPv6 Header Version = 6 Traffic Class = 0 FlowLabel = 0 PayloadLength = 528 NextHeader = 44 (Fragment Header) SourceAddress = Tester Link Local Address DestinationAddress = Target Link Local Address
Fragment Header NextHeader = 58 (ICMP) FragmentOffset = 0 MFlag = 1 Identification = 32bit (Automatic generation)
Payload data = 520 octets from the head of ICMP Echo request
Echo Request Data (2nd fragment) is:
IPv6 Header Version = 6 Traffic Class = 0 FlowLabel = 0 PayloadLength = 520 NextHeader = 44 (Fragment Header) SourceAddress = Tester Link Local Address DestinationAddress = Target Link Local Address
Fragment Header NextHeader = 58 (ICMP) FragmentOffset = 65 MFlag = 0 Identification = 32bit (Automatic generation)
Payload data = 512 octets from the back of ICMP Echo request
PASS: Echo Reply Received
IPv6 Header Version = 6 Traffic Class = 0 FlowLabel = 0 PayloadLength = 1032 NextHeader = 58 (ICMP) SourceAddress = Target Link Local Address Destination Address = Tester Link Local Address
ICMP Echo Reply Type = 129 (Echo Reply) Code = 0 Checksum = (auto) Identifier = (same as Echo Request) SequenceNumber = (same as Echo Request) PayloadData = (same as Echo Request)
RFC2460
4.5 Fragment Header
The Fragment header is used by an IPv6 source to send a packet larger than would fit in the path MTU to its destination. (Note: unlike IPv4, fragmentation in IPv6 is performed only by source nodes, not by routers along a packet's delivery path -- see section 5.) The Fragment header is identified by a Next Header value of 44 in the immediately preceding header, and has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Reserved | Fragment Offset |Res|M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header 8-bit selector. Identifies the initial header type of the Fragmentable Part of the original packet (defined below). Uses the same values as the IPv4 Protocol field [RFC-1700 et seq.].
Reserved 8-bit reserved field. Initialized to zero for transmission; ignored on reception.
Fragment Offset 13-bit unsigned integer. The offset, in 8-octet units, of the data following this header, relative to the start of the Fragmentable Part of the original packet.
Res 2-bit reserved field. Initialized to zero for transmission; ignored on reception.
M flag 1 = more fragments; 0 = last fragment.
Identification 32 bits. See description below.
In order to send a packet that is too large to fit in the MTU of the path to its destination, a source node may divide the packet into fragments and send each fragment as a separate packet, to be reassembled at the receiver.
For every packet that is to be fragmented, the source node generates an Identification value. The Identification must be different than that of any other fragmented packet sent recently* with the same Source Address and Destination Address. If a Routing header is present, the Destination Address of concern is that of the final destination.
* "recently" means within the maximum likely lifetime of a packet, including transit time from source to destination and time spent awaiting reassembly with other fragments of the same packet. However, it is not required that a source node know the maximum packet lifetime. Rather, it is assumed that the requirement can be met by maintaining the Identification value as a simple, 32- bit, "wrap-around" counter, incremented each time a packet must be fragmented. It is an implementation choice whether to maintain a single counter for the node or multiple counters, e.g., one for each of the node's possible source addresses, or one for each active (source address, destination address) combination.
The initial, large, unfragmented packet is referred to as the "original packet", and it is considered to consist of two parts, as illustrated:
original packet:
+------------------+----------------------//-----------------------+ | Unfragmentable | Fragmentable | | Part | Part | +------------------+----------------------//-----------------------+
The Unfragmentable Part consists of the IPv6 header plus any extension headers that must be processed by nodes en route to the destination, that is, all headers up to and including the Routing header if present, else the Hop-by-Hop Options header if present, else no extension headers.
The Fragmentable Part consists of the rest of the packet, that is, any extension headers that need be processed only by the final destination node(s), plus the upper-layer header and data.
The Fragmentable Part of the original packet is divided into fragments, each, except possibly the last ("rightmost") one, being an integer multiple of 8 octets long. The fragments are transmitted in separate "fragment packets" as illustrated:
original packet:
+------------------+--------------+--------------+--//--+----------+ | Unfragmentable | first | second | | last | | Part | fragment | fragment | .... | fragment | +------------------+--------------+--------------+--//--+----------+ fragment packets:
+------------------+--------+--------------+ | Unfragmentable |Fragment| first | | Part | Header | fragment | +------------------+--------+--------------+
+------------------+--------+--------------+ | Unfragmentable |Fragment| second | | Part | Header | fragment | +------------------+--------+--------------+ o o o +------------------+--------+----------+ | Unfragmentable |Fragment| last | | Part | Header | fragment | +------------------+--------+----------+
Each fragment packet is composed of:
(1) The Unfragmentable Part of the original packet, with the Payload Length of the original IPv6 header changed to contain the length of this fragment packet only (excluding the length of the IPv6 header itself), and the Next Header field of the last header of the Unfragmentable Part changed to 44.
(2) A Fragment header containing:
The Next Header value that identifies the first header of the Fragmentable Part of the original packet.
A Fragment Offset containing the offset of the fragment, in 8-octet units, relative to the start of the Fragmentable Part of the original packet. The Fragment Offset of the first ("leftmost") fragment is 0.
An M flag value of 0 if the fragment is the last ("rightmost") one, else an M flag value of 1.
The Identification value generated for the original packet.
(3) The fragment itself.
The lengths of the fragments must be chosen such that the resulting fragment packets fit within the MTU of the path to the packets' destination(s).
At the destination, fragment packets are reassembled into their original, unfragmented form, as illustrated:
reassembled original packet:
+------------------+----------------------//------------------------+ | Unfragmentable | Fragmentable | | Part | Part | +------------------+----------------------//------------------------+
The following rules govern reassembly:
An original packet is reassembled only from fragment packets that have the same Source Address, Destination Address, and Fragment Identification.
The Unfragmentable Part of the reassembled packet consists of all headers up to, but not including, the Fragment header of the first fragment packet (that is, the packet whose Fragment Offset is zero), with the following two changes:
The Next Header field of the last header of the Unfragmentable Part is obtained from the Next Header field of the first fragment's Fragment header.
The Payload Length of the reassembled packet is computed from the length of the Unfragmentable Part and the length and offset of the last fragment. For example, a formula for computing the Payload Length of the reassembled original packet is:
PL.orig = PL.first - FL.first - 8 + (8 * FO.last) + FL.last
where PL.orig = Payload Length field of reassembled packet. PL.first = Payload Length field of first fragment packet. FL.first = length of fragment following Fragment header of first fragment packet. FO.last = Fragment Offset field of Fragment header of last fragment packet. FL.last = length of fragment following Fragment header of last fragment packet.
The Fragmentable Part of the reassembled packet is constructed from the fragments following the Fragment headers in each of the fragment packets. The length of each fragment is computed by subtracting from the packet's Payload Length the length of the headers between the IPv6 header and fragment itself; its relative position in Fragmentable Part is computed from its Fragment Offset value.
The Fragment header is not present in the final, reassembled packet.
The following error conditions may arise when reassembling fragmented packets:
If insufficient fragments are received to complete reassembly of a packet within 60 seconds of the reception of the first-arriving fragment of that packet, reassembly of that packet must be abandoned and all the fragments that have been received for that packet must be discarded. If the first fragment (i.e., the one with a Fragment Offset of zero) has been received, an ICMP Time Exceeded -- Fragment Reassembly Time Exceeded message should be sent to the source of that fragment.
If the length of a fragment, as derived from the fragment packet's Payload Length field, is not a multiple of 8 octets and the M flag of that fragment is 1, then that fragment must be discarded and an ICMP Parameter Problem, Code 0, message should be sent to the source of the fragment, pointing to the Payload Length field of the fragment packet.
If the length and offset of a fragment are such that the Payload Length of the packet reassembled from that fragment would exceed 65,535 octets, then that fragment must be discarded and an ICMP Parameter Problem, Code 0, message should be sent to the source of the fragment, pointing to the Fragment Offset field of the fragment packet.
The following conditions are not expected to occur, but are not considered errors if they do:
The number and content of the headers preceding the Fragment header of different fragments of the same original packet may differ. Whatever headers are present, preceding the Fragment header in each fragment packet, are processed when the packets arrive, prior to queueing the fragments for reassembly. Only those headers in the Offset zero fragment packet are retained in the reassembled packet.
The Next Header values in the Fragment headers of different fragments of the same original packet may differ. Only the value from the Offset zero fragment packet is used for reassembly.
perldoc V6evalTool